The present invention relates to a demodulator that demodulates a frequency-modulated radio broadcast signal by digital signal processing.
Frequency-modulated (FM) radio broadcast receivers have traditionally employed analog demodulator circuits, typically comprising a limiter and a quadrature detector. With the increasing miniaturization of digital integrated circuits, however, it has become advantageous to employ digital FM demodulator circuits, especially in receivers that also receive digital broadcast signals.
In an FM radio broadcast receiver with a digital FM demodulator, the radio-frequency (RF) signal received at the antenna is down-shifted to an intermediate frequency (IF), then passed through an IF limiting amplifier that outputs a constant-amplitude IF signal. The constant-amplitude IF signal is converted to a digital signal, demodulated, then converted back to an analog signal for output to a loudspeaker.
Since the IF limiting process introduces unwanted harmonic components into the IF signal, the output of the IF limiting amplifier is passed through a low-pass pre-filter to reject the harmonics before analog-to-digital conversion. The pre-filter must combine ample attenuation of the harmonic components with an extremely flat passband characteristic, with substantially no ripple, to avoid re-introducing amplitude variations into the IF signal. It is difficult to satisfy both of these requirements in a filter of the type that can be realized in a semiconductor integrated circuit. If the pre-filter is disposed in a semiconductor integrated circuit, which is necessary for miniaturization of the receiver circuitry, amplitude variations cannot be avoided, but these amplitude variations distort the demodulated signal.
A further discussion of this problem will be given in the detailed description of the invention.
An object of the present invention is to provide a digital FM demodulator that compensates for unwanted amplitude variations generated in receiver components preceding the demodulator, thereby relaxing the performance requirements of these components.
The invented digital FM demodulator has means for performing the following processes:
converting a digital FM input signal to a demodulated signal;
detecting the amplitude of the digital FM input signal and generating a corresponding amplitude signal; and
adjusting the amplitude of the demodulated signal according to the amplitude signal, thereby compensating for variations in the amplitude of the digital FM input signal.
Since the digital FM demodulator compensates for amplitude variations, receiver components that process the FM signal before input to the digital FM demodulator can be permitted to generate amplitude variations that could not be tolerated with a conventional digital FM demodulator.
The means for converting the digital FM input signal to a demodulated signal may operate by delaying the digital FM input signal and multiplying the digital FM input signal by the delayed signal. The demodulated signal can be obtained by comparatively simple processing in this way.
The amplitude signal can be obtained by taking the sum of the squares of the digital FM input signal and the delayed signal. The amplitude signal can be calculated efficiently in this way.
The means for adjusting the amplitude of the demodulated signal may use a polynomial approximation to calculate the reciprocal of the amplitude signal. The reciprocal can thus be calculated by comparatively simple processing.
The demodulated signal and amplitude signal may be decimated before the amplitude of the demodulated signal is adjusted. The necessary amount of adjustment processing is thereby reduced.
The invented digital FM demodulator may also have means for multiplying the digital FM input signal by a coefficient to reduce amplitude variations before demodulation is performed. The necessary adjustments to the amplitude of the demodulated signal can then be confined to a comparatively small range, facilitating the computations performed in the adjustment process.
The invented digital FM demodulator may also have means for applying an arcsine correction to the demodulated signal after amplitude adjustment. The arcsine correction improves the linearity of the demodulated signal.